The present invention relates to data processing systems and, more specifically, to automated data storage and retrieval systems which comprise a library of media elements as well as one or more drives for reading from and writing to the media elements in the library.
Magnetic tape cartridges, magnetic disks, and optical disks are all widely used as peripheral memory storage devices for computer systems. Large computer systems often operate in conjunction with external libraries having dozens of such media elements as well as the media element readers used to retrieve and record data. Although originally such media elements were selected and loaded manually, automated libraries were developed to expedite the handling of the media. These systems include means for accessing a desired media element, retrieving it from its storage position, and loading it into an appropriate reader. More recently, instruction sets have been created which define a communication protocol between the host computer system and the library. The instruction sets include commands to move media to different locations within the library and, of course, to load media into drives, place the media in a particular logical position, and read or write from and to specified regions of the media.
As data storage requirements for computer systems have increased front megabytes to gigabytes to terabytes, the development of automated media libraries has received considerable attention. Some embodiments of such libraries comprise a small number of media elements, six or ten being typical, and one or two drives housed in a single enclosure. Cabinet and even room sized systems have also been developed which hold a much larger number of media elements and drives, and which further comprise robotic arms, often translatable on all three axes, which remove media elements from storage and place them in drives. Furthermore, due to the increasing use of wide-area-networks, interconnected library systems forming a single dispersed database have become more common.
However, one problem with traditional library systems is that only one host computer may be effectively attached to a library at a time. This is a result of the fact that host computers attached to the library keep a log of the contents of the media contained in the library. If a second computer is attached to the same library, the second computer may modify the contents of the library which is being relied upon by the first computer. This poses a problem for network system administrators which prevents the use of cost effective large library systems. Typically, separate file servers in a network will have a dedicated small media library for use by the particular host file server. Data transfer requests from network clients must be routed to the library through the appropriate host. Data storage flexibility is thus reduced and data storage capacity is rendered more expensive because multiple physical libraries are required.
Accordingly, a need exists in the art for improvements in the distribution of data, storage capacity among a plurality of computers such as are present in a computer network.